1. Field of the Invention
This invention relates to a semiconductor memory device and a memory card including the semiconductor memory device. More particularly, this invention relates to a nonvolatile semiconductor memory device including MOS transistors, each having a floating gate and a control gate.
2. Description of the Related Art
A NAND flash memory has been known as a data storage memory for use in a digital camera or the like. A NAND flash memory uses FN (Fowler-Nordheim) tunneling to write and erase data.
Another known flash memory is a NOR flash memory which writes and erases data by using FN tunneling. A NOR flash memory has been written in, for example, Ditewig T., et al., “An Embedded 1.2-V Read Flash Memory Module in a 0.18-μm Logic Process,” Solid-State Circuits Conference, 2001 Digest of Technical Papers ISSCC. 2001 IEEE International 5–7, February 2001, pp. 34–35, 425. Unlike a flash memory which exchanges electrons using hot electrons, the NOR flash memory described in the reference is such that a memory cell has one memory cell transistor and two select transistors. Hereinafter, this type of flash memory is referred to as a 3Tr-NAND flash memory.
Furthermore, in recent years, a flash memory combining the features of the NOR flash memory and the NAND flash memory has been proposed in, for example, Wei-Hua Liu, “A 2-Transistor Source-select (2TS) Flash EEPROM for 1.8-V-Only Application,” Non-Volatile Semiconductor Memory Workshop 4.1, 1997. A flash memory of this type has memory cells, each including one memory cell transistor and one select transistor. Hereinafter, this type of flash memory is referred to as a 2Tr flash memory.
However, with the conventional flash memory, it may take a long time for the gate potential of a memory cell to settle down because of the influence of adjacent electrodes coupling with one another.